Non-sintered electrode and method of manufacturing same

ABSTRACT

A metal sheet ( 1 ) which constitutes a non-sintered type electrode support is processed to have minute irregularities on its surface. The irregularities are formed by a mechanical method such that protrusions ( 9 ) and indentations ( 8 ) are configured with a center-to-center pitch (P) in the range of from 50 to 300 μm and such that the apparent thickness after processing is at least three times as large as the unprocessed material thickness.

TECHNICAL FIELD

The present invention relates to a non-sintered type electrode for usein storage batteries (secondary batteries), such as alkaline storagebattery, lithium ion storage battery and polymer lithium storagebattery.

BACKGROUND ART

Various types of electrodes are known for the storage battery (secondarybattery). Non-sintered electrodes are defined to be the electrodes thatcan be obtained without sintering and are fabricated by coating orpressing an active material for battery or active material retainingmedium onto an electrode support.

The process of fabricating a nickel positive electrode of alkalinestorage battery as one example of sintered electrodes is describedbelow. A microporous sintered plaque obtained by sintering nickel powderis impregnated with an aqueous solution of nickel nitrate or the likethereby to add nickel salt, and, after drying, the sintered plaque isimmersed in caustic alkali aqueous solution to convert the nickel saltto nickel hydroxide. This method has the disadvantage that the processis complicated and the filling density of nickel hydroxide as an activematerial is reduced in comparison with the non-sintered electrodedescribed later. In spite of this disadvantage, this electrode has anexcellent high-rate discharge characteristic and a long cycle life, andfinds wide application in a variety of field.

A method called pocket system was previously used for the manufacture ofnon-sintered electrode, while paste applying or pressing methods havebecome the mainstream in the recent years. In the paste applying method,the active material for battery itself or active material retainingmedium is prepared into a paste with water or an organic solution, thispaste is applied on an electrode support and dried. In the pressingmethod, on the other hand, the active material for battery or activematerial retaining medium in the form of powder directly fills theelectrode support by being pressed thereonto with a press machine or thelike.

A variety of materials are used for these non-sintered electrodes,including metal foil, perforated metal plate, metal net, expanded metal,foamed porous metal material and the like. These materials have beenapplied with their properties and forms varied-accordingly so as to suiteach battery system.

For example, foamed nickel porous material having a three dimensionallyreticulated structure has been commonly used as the electrode supportfor the positive electrode of non-sintered type used in alkaline storagebatteries such as nickel-hydrogen storage battery or nickel-cadmiumstorage battery. For the negative electrode, on the other hand, punchedmetal has been mainly employed.

The method using foamed nickel porous material is a simple method ofelectrode production. Further, the availability of a foamed nickelporous material of high porosity makes it possible to fill it withnickel hydroxide to a high density and therefore a high-capacity batterycan be produced. The foamed nickel porous material, however, needs to beproduced by electroplating and therefore has the disadvantage of highmaterial cost.

In view of this, a non-sintered electrode is under development using alow-cost punched metal or expanded metal in place of the foamed nickelporous material as an electrode support. These electrode supports haveno three-dimensional structure unlike the sintered plaque or thefoamed-nickel porous material. As a result, an electrode made of theseelectrode supports has a low ability to hold an active material and theactive material is liable to fall off during electrode fabrication orrepeated charging and discharging. Further, due to the low electronicconductivity in the electrode thickness direction and a poor electrodecharacteristic, which is a serious hindrance to the application in thenickel positive electrode of alkaline storage battery, these electrodesupports find no practical applications except for special types ofelectrodes.

Japanese Laid-Open Patent Application No.7-130370 and No.9-7603 disclosetechnologies for improvement of these electrode supports. The electrodesupport according to JP Laid-Open No.7-130370 is constructed of flatmetal sheet or flat metal foil and thus is weak in the adhesion betweenthe active material layer and electrode support. Separation of activematerial from electrode support occurs particularly in the applicationas the electrode of a storage battery due to changes in volume of theactive material caused by repeated charging and discharging. Currentcollecting ability decreases accordingly, as a result of which thebattery characteristics are deteriorated.

As a countermeasure for this drawback, formation of minuteirregularities using metal powder is proposed in JP Laid-Open No.9-7603.The adhesion between the active material layer and the electrode supportis thereby improved. However, the production cost of electrode supportswill be raised because of sintering in an inactive gas atmosphere orelectroplating methods required for forming the minute irregularitieslayer.

In both of the above electrode supports, furthermore, in the case ofcorrugating the electrode supports so that they have a three-dimensionalstructure, they are more subjected to deformation and elongation duringthe compressing process for filling the active material at a highdensity. As a result, cracks or rupture occur in the electrode support,which leads to troubles such as decrease in current collecting abilityof the electrode and micro-short circuit when assembled as a battery.Moreover, the above mentioned deformation and elongation of theelectrode support also set a limit to the high-density filling and abattery of large discharge capacity cannot be obtained.

Meanwhile, efforts have been made to improve the electrodecharacteristics such as retaining property of active material andelectronic conductivity for the negative electrode of alkaline storagebattery (cadmium electrode or hydrogen-absorption alloy electrode),using inexpensive punched metal or expanded metal while exploiting theiradvantages. Further improvement is desired to achieve a more efficienthigh-rate discharge characteristic and a longer cycle life, which arestill unsatisfactory in these negative electrodes.

These demands are also applicable to other types of batteries such asfor example lithium ion storage battery or polymer lithium storagebattery. There has generally been a desire for an electrode using a lowcost electrode support while exhibiting excellent performance.

The above-mentioned method of electrode production using a punched metalor expanded metal as an-electrode support has the advantage that apowder of active material made into a paste with a solution of a highpolymer binder and a conductive powder is coated and dried on theelectrode support and thus the electrode can be easily produced. Theadhesion between the metal substrate acting as the electrode support andthe active material layer is generally weak so that the active materialis liable to peel off from the metal substrate in an application usingthe electrode for batteries. In the case where the electrode supportacts as a current collector, the electrical resistance of the electrodeincreases thereby causing a reduced discharge voltage and dischargecapacity. In order to solve this problem, adding a great amount ofbinder to the active material layer suppresses the separation. Theresultant reduced reactivity of the active material, however, has anadverse effect on the discharge characteristic.

In a method for strengthening the adhesion between the electrode supportand the active material layer, a thermoplastic resin layer functioningas a binder is formed on the surface of the electrode support. Then, theactive material is coated on the thermoplastic resin layer and theelectrode is heated, to improve the adhesion between the electrodesupport and the active material layer. This method, however, has adisadvantage that a resin insulating layer is formed between the metalelectrode support and the active material layer with the result that thecurrent collecting characteristic of the electrode is reduced, therebyreducing the reactivity of the electrode.

As described above, these problems are difficult to solve when acomparatively flat metal substrate-is used as an electrode support.

Accordingly, an object of the present invention is to provide allimprovement in a non-sintered type electrode with an active material oractive material retaining medium coated or pressed on an electrodesupport, in order to achieve an improved adhesion and improvedelectronic conductivity between the active material layer and theelectrode support, while maintaining the advantage of low material cost.

Another object of the present invention is to provide an improvednon-sintered electrode which is favorably used as the nickel electrodeof alkaline storage batteries such as nickel hydrogen storage batteryand nickel-cadmium storage battery, the hydrogen-absorption alloyelectrode which uses hydrogen-absorption alloy powder, and the cadmiumelectrode, as well as for the electrodes of lithium ion storage batteryand polymer lithium storage battery.

DISCLOSURE OF THE INVENTIOIN

In order to achieve the above objects, the present invention provides anon-sintered type electrode comprising an electrode support made of ametal sheet having minute surface irregularities on which is coated orpressed an active material for battery or an active material retainingmedium, characterized in that said surface irregularities are formed bya mechanical method such that protrusions and indentations areconfigured with a center-to-center pitch in the range of from 50 to 300μm, and such that the apparent thickness after forming the surfaceirregularities is at least three times as large as the thickness beforethe formation of the surface irregularities.

The protrusions and indentations should preferably be formed in asubstantially tapered shape such as conical shape, but may also beformed in hemispheric shape.

The apparent thickness of the electrode support after forming thesurface irregularities should preferably be 200 μm or more and inparticular 400 μm or more, and should be at least five times as large asthe thickness before the formation of the surface irregularities.

The electrode support may be constructed of a punched metal or anon-punched metal sheet, of which material thickness before theformation of the surface irregularities should preferably be in therange of from 10 to 80 μm and in particular from 20 to 60 μm. Theelectrode support should preferably be made of nickel sheet, but may bealso constructed of steel sheet or nickel-plated steel sheet.

The center-to-center pitch of the protrusions and indentations shouldpreferably be in the range of from 50 to 300 μm, and in particular 100to 200 μm.

In addition to the above construction, it is preferable that the metalsheet with the surface irregularities has innumerable minute holesformed by piercing through the tops of the protrusions. In particular,it is preferable to construct the electrode support by forming suchminute holes in a non-punched metal sheet.

The protrusions and indentations of the surface irregularities shouldpreferably be formed either at random or in order with the respectivenumbers of protrusions and indentations in ranges of from 80 to 20% andfrom 20 to 80% per unit area. The arrangement of the protrusions andindentations may be such that they are formed alternately in onedirection or in both longitudinal and transverse directions.

Owing to the above construction, the electrode support according to thepresent invention has an advantageous feature of low manufacturing cost,since it can be fabricated simply by a mechanical method just likecommon punched metal which is obtained from a nickel-plated steel sheetby a mechanical process. In addition to this, the electrode support ofthe present invention is formed with minute surface irregularities withthe center-to-center pitch between adjacent protrusions and indentationsin the range of from 50 to 300 μm, whereby the apparent thickness of theelectrode support is increased to at least three times as large as theunprocessed material thickness. Thanks to this drastic transformation ofthe structure into three-dimensional form, the active material retainingability of the electrode support is remarkably improved owing to theminute surface irregularities, and separation of the active materiallayer from the electrode support is suppressed in comparison with atwo-dimensional electrode support such as punched metal. The electronicconductivity in the electrode thickness direction is also enhanced. Theutilization rate of active material is accordingly increased, wherebythe battery of higher capacity can be obtained.

There might have been a trouble that such machining process causes thetensile strength of the electrode support to decrease and that the yieldof product is reduced accordingly, but this problem was avoided byproviding a strip-form solid portion in the electrode support where nopunched holes or irregularities are provided.

The electrode support according to the present invention as describedabove can constitute a non-sintered electrode of excellent quality.Specifically, the above technologies can be applied for the fabricationof a nickel positive electrode of an alkaline storage battery. In thatcase, the active material should preferably comprise nickel hydroxidepowder of which surface is coated with cobalt oxide of higher order atthe rate of 2 to 10 wt % in relation to 100 wt % of nickel hydroxide, oralternatively, cobalt oxide of higher order or nickel may be mixed inthe nickel hydroxide powder. In the case where the cobalt oxide ofhigher order is solely used, it should be contained at the rate of 2 to10 wt % in relation to 100 wt % of nickel hydroxide, whereas if nickelis solely used, it should be contained at the rate of 10 to 25 wt % inrelation to 100 wt % of nickel hydroxide. A nickel positive electrode ofextremely high utilization rate can be thereby obtained.

Furthermore, the above technologies can be also applied for thefabrication of a hydrogen-absorption alloy negative electrode of analkaline storage battery. In that case, the active material retainingmedium may comprise hydrogen-absorption alloy powder alone, or aconductive agent selected from nickel, copper, and carbon, may becontained therein at the rate of 0.5 to 10 wt % in relation to 100 wt %of hydrogen-absorption alloy powder. Such nickel or copper may be coatedon the surface of the hydrogen-absorption alloy powder in the sameamount as mentioned above. A hydrogen-absorption alloy negativeelectrode of extremely high utilization rate and excellent high-ratedischarge characteristic can be thereby obtained.

It is also possible to apply the electrode support of the presentinvention to a lithium ion storage battery or lithium polymer storagebattery, and to improve the electrode characteristics thereof byselecting the materials appropriately.

The present invention also provides, in order to achieve the above saidobjects, a method of manufacturing a non-sintered type electrodecharacterized in that minute irregularities are formed on the surface ofa metal sheet by press-machining using metal molds such that protrusionsand indentations are configured with a center-to-center pitch in therange of from 50 to 300 μm and such that the apparent thickness afterforming the surface irregularities is at least three times as large asthe thickness before the formation of the surface irregularities,thereby fabricating an electrode support, and an active material forbattery or an active material retaining medium is coated or pressed onthe surface of the thus obtained electrode support.

In the above process of press-machining the metal sheet with metalmolds, minute holes should preferably be formed simultaneously with theformation of the surface irregularities by piercing through the tops ofthe protrusions. Also, it is preferable that each of the protrusions andindentations is formed in a substantially tapered shape.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing one example of metal molds forfabricating the electrode support according to one embodiment of thepresent invention;

FIGS. 2A and 2B are cross sectional views illustrating a rolled steelsheet fabricated using the metal molds of FIG. 1, wherein FIG. 2A showsthe case where protrusions and indentations are formed, and FIG. 2Bshows the case where the rolled steel sheet of FIG. 2A has been furtherpressed by the molds until the protrusions and indentations are piercedthrough;

FIG. 3 is a top plan view illustrating a rolled steel sheet whereinprotrusions and indentations have been formed in rows;

FIG. 4 is a top plan view illustrating a rolled steel sheet whereinprotrusions and indentations have been formed in a zigzag fashion;

FIG. 5 is a top plan view illustrating a rolled steel sheet whereinprotrusions and indentations have been formed alternately in rows; and

FIG. 6 is a top plan view illustrating a rolled steel sheet whereinprotrusions and indentations have been formed alternately in a zigzagfashion.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention in the form of specificexamples are hereinafter described with reference to the accompanyingdrawings.

EXAMPLE 1

A rolled steel sheet (SPCE) having a thickness of 50 μm of which surfaceis plated with nickel to the thickness of 2˜3 μm was used. FIG. 1 is alongitudinal sectional view showing a construction of a mold used forforming minute irregularities in the metal sheet to be used as theelectrode support. An upper mold 2 has pointed projections 3 of coneshape and recesses 4 corresponding to projections formed on a lower mold5, which comprises projections 6 and recesses 7 similarly to the uppermold 2. Reference numeral 1 represents the rolled steel sheet.

The rolled steel sheet 1 is arranged between the molds such as to befixed in position at its periphery. When the upper mold 2 and lower mold5 are pushed together, the rolled steel sheet 1 is stretched by means ofthe projections and recesses of the molds. If the molds are furtherpressed to each other, the rolled steel sheet 1 will be perforated. Itis thus possible to adjust as required the rate of stretch, the shape ofpunched-out holes and the rate of perforation, by varying theconfiguration of the upper and lower molds or the distance of the moldspressed against to each other.

FIG. 2 is a cross sectional view showing a rolled steel sheet afterbeing processed using the above molds. FIG. 2A shows the case where noholes have been made, whereas in FIG. 2B, holes 1 a have been formed.

Using various molds respectively having different center-to-centerpitches (P shown in FIG. 2A), perforated metal sheets ofthree-dimensional structure having innumerable minute holes 1 a a˜e wereproduced, their inter-hole pitches ranging from 50˜400 μm. There was noconspicuous difference in the performance of electrodes between themetal sheets processed to have surface irregularities without holes 1 aand those with the holes 1 a. The metal sheets with minute holes 1 awere used as samples in the experiments described later. The pattern ofprotrusions and indentations was such that the central points of theprotrusions and indentations were arranged in one line in a longitudinaldirection as shown in FIGS. 3 and 4. The metal sheet was successivelyprocessed from left to right in a lengthwise direction of the strip ofmetal sheet, the reference numeral 8 in FIG. 3 representing indentationsformed by pressing the metal sheet downwards, and 9 showing protrusionsformed by upward pressing.

All of the perforated metal sheets a˜e were produced with the patternshown in FIG. 3. Comparative sample c′ was also prepared, of which P isthe same 200 μm as that of the perforated metal sheet c, with thepattern shown in FIG. 4, wherein center points of adjacent protrusionsand indentations are offset to each other in a transverse direction.

Formation of protrusions and indentations with P of less than 50 μm willnecessitate micro-machining of the molds, which leads to high productioncost. Moreover, the apparent thickness will not be sufficientlyincreased by the formation of irregularities. In order to achieveeffectively the benefits of the present invention, which is theprovision of an electrode support with enhanced conductivity in thethickness direction at low cost, the inter-hole pitch should be greaterthan 50 μm.

Furthermore, a normal punched metal obtained by punching holes of thediameter of 1.2 mm in a nickel-plated rolled steel sheet having athickness of 80 μm with the center-to-center distance between adjacentholes of 2.4 mm, was pressed with the molds of FIG. 1 thereby to obtaina perforated metal sheet f with protrusions and indentations in thearrangement shown in FIG. 3 with the inter-hole pitch of 100 μm. Thisperforated metal sheet f and a normal punched metal g without surfaceirregularities were used for the evaluation of electrodes describedlater.

Nickel-plated rolled steel sheets having a thickness ranging from 10 to100 μm were pressed with the same molds as those used for producing theabove metal sheet c to obtain metal sheets h˜l as the comparativeexamples with respect to the metal sheets having the thickness of 50 μmbefore processing, thereby to analyze how the metal sheet thicknessbefore processing affects the performance of battery.

Furthermore, a nickel-plated rolled steel sheet of which raw materialthickness is 50 μm was formed with protrusions and indentations in apattern shown in FIG. 5, wherein protrusions and indentations arealternately formed in both longitudinal and transverse directions, withthe inter-hole pitch P of the same 200 μm as that of metal sheet c,thereby obtaining a perforated metal sheet m. Similarly, a perforatedmetal sheet n was produced with the pattern shown in FIG. 6.

Besides, a metal sheet of pure nickel having a thickness of 50 μm waspressed with the molds of FIG. 1 in the pattern of FIG. 3 with theinter-hole pitch of 200 μm, so as to obtain a perforated metal sheet o.

The specification of these electrode supports is summarized in Table 1.

TABLE 1 Raw Inter- Apparent Perfo- material hole thickness rated thick-Pitch after metal ness (P) Punched processing sheet (μm) Material (im)holes (μn) Remarks a 50 Nickel-  50 Absent 250 plated rolled steel b 50ditto 100 ditto 400 c 50 ditto 200 ditto 500 c′ 50 ditto 200 dittb 500 d50 ditto 300 ditto 500 e 50 ditto 400 ditto 500 f 80 ditto 100 Present400 g 80 ditto — Present (80) Normal product h 10 ditto 200 500 i 30ditto 200 ditto 500 j 60 ditto 200 ditto 500 k 80 ditto 200 ditto 500 l100 ditto 200 ditto 500 m 50 ditto 200 ditto 500 FIG. 5 n 50 ditto 200ditto 500 FIG. 6 o 50 Pure 200 ditto 500 nickel

These perforated metal sheets a˜o were used to prepare paste-coated typenickel positive electrodes.

First, 100 g of nickel hydroxide powder were mixed into a paste formwith 10 g of cobalt powder, 55 g of 3 wt % aqueous solution ofcarboxymethyl cellulose and 5 g of 48 wt % styrene-butadiene rubberaqueous dispersion. Each perforated metal sheet shown in Table 1 waspassed through a bath containing this paste to coat the paste on bothfaces of the perforate metal sheet. The coated perforated metal sheetswere then passed through a stainless-steel slit to reduce them to apredetermined thickness. Then the metal sheets were dried and compressedto prepare a coated-nickel positive electrode having a thickness of from0.63 mm to 0.65 mm.

Next, these nickel electrodes were cut into rectangles (i.e., 38 mm×220mm), and their weights were measured. The electrochemical theoreticalcapacity calculated from the amount of nickel hydroxide contained in theelectrode thus obtained was in the range of from 2674 mAh to 3092 mAh.

Each of these nickel positive electrodes was combined with a well-knownrare earth-nickel based hydrogen-absorption alloy negative electrode anda separator made of unwoven fabric of polypropylene processed to acquirehydrophilicity to configure a cylindrical sealed battery (size C) havinga nominal capacity of 2.8 Ah. An aqueous solution of potassium hydroxide(3 wt %) dissolved with lithium hydroxide 30 g/l was used in an amountof 6 ml per cell as the electrolyte.

Batteries A˜O using the nickel positive electrodes obtained respectivelyfrom the perforated metal sheets a˜o shown in Table 1 were thusprepared.

The batteries constructed as described above were charged for 15 hoursat 0.1C., and left for one hour and thereafter discharged at 0.2C. untilthe battery voltage decreased to 1.0V. Three cycles of this test wererepeated under the same conditions. Next, under similar chargingconditions, the fourth-cycle test was conducted with a discharge currentof 0.5C., and the fifth-cycle test was conducted with a dischargecurrent of 1C., to compare the discharge characteristics. Also, for thesixth and subsequent cycles, the cycle life test was conducted bycharging at 0.3C. for four hours and discharging at 0.5C. until thebattery voltage decreased to 1V to compare the structure of the nickelpositive electrode and the cycle life characteristics. The results areshown in Table 2.

TABLE 2 Utilization Discharge Discharge of active capacity capacityDischarge Theoretical material at at 5th at 100th capacity at Bat-capacity 3rd cycle cycle cycle 300th cycle tery (mAh) (%) (mAh) (mAh)(mAh) A 2870 97.4 2516 2063 1762 B 2883 97.5 2586 2483 2276 C 2902 98.02645 2486 2195 C′ 2898 98.0 2649 2503 2219 D 2914 95.8 2540 1981 1524 E2933 92.1 2296 1492 1148 F 2950 97.8 2655 2575 2390 G 2965 85.1 22831096 753 H 3092 95.3 2659 2420 1912 I 3028 96.8 2634 2423 2134 J 286298.1 2633 2448 2185 K 2793 98.0 2570 2313 1953 L 2674 98.3 2460 21651673 M 2913 99.8 2680 2573 2278 N 2901 101.1 2669 2602 2322 O 2927 98.82722 2586 2286

In comparing the charge and discharge characteristics of batteries A˜Eshown in Table 2, it was found that the utilization of the activematerial could be made more than 95% if the inter-hole pitch P was inthe range of 50-300 μm, and that there was a trend of decline in theutilization when the P was greater than 300 μm. It is assumed that asmaller P will generally help enhance the electronic conductivitybetween the active material and the electrode substrate and thus willcause the utilization to increase. The effects of smaller P were alsofound in the large-current discharge characteristic at the 5th cycle.The result of the cycle life test also showed that the electrodesupports with P in the above mentioned range have a longer life. Theeffects were remarkable especially in the batteries B and C., of which Pwere 100 μm and 200 μm, respectively.

In battery F, an electrode support obtained by providing surfaceirregularities according to the present invention to a normal punchedmetal was used with the same P as that of the electrode support ofbattery B. From the fact that, as compared with battery B, the effectsof the present invention were recognized in battery F, too, it is seenthat the same effects can be achieved by providing surfaceirregularities in a normal punched metal.

In battery G, on the other hand, wherein the electrode support g ofpunched metal with no surface irregularities constitute the electrode,the utilization of the active material and cycle life characteristicwere extremely low. It is thus construed that providing the electrodesupport with a three-dimensional structure by forming minute protrusionsand indentations on the surface of electrode support is effective toincrease the electronic conductivity in the thickness direction oftheelectrode and the retaining ability of the active material, and thusto enhance the performance of electrode. The separation of the activematerial from the electrode support is caused by generation of oxygengas from the positive electrode under an overcharged state and thechange in volume of the active material due to charging and discharging.The longer cycle life of battery observed in the above test can beparticularly attributed to the fact that the active material did noteasily peel off from the electrode support.

As set forth above, electrodes of high performance can be obtained byforming minute protrusions and indentations on the surface of metalsheet, since this causes the contact area with the active material toincrease, and the active material retaining ability of the electrodesupport to improve.

Meanwhile, it is seen from the comparison between batteries A and B thatthe apparent thickness of metal sheets after processing should be threetimes as large as the thickness before processing and more than 200 μm(preferably five times as large and more than 400 μm) to achieve theeffects of longer life.

With respect to batteries H˜L using the electrode supports which wereprepared by pressing nickel-plated rolled steel sheets having athickness ranging from 10 to 100 μm with a fixed inter-hole pitch (P) of200 μm for the purpose of analyzing the influence of thickness ofunprocessed metal sheet, when compared with battery C, the utilizationof the active material and large-current discharge characteristic wereboth improved in the case where the nickel-plated rolled steel beforeprocessing was of the thickness of 50˜60 μm or more. In the cycle lifecharacteristic, on the other hand, the peak value appeared in the casewhere the thickness of unprocessed metal sheet was more or less 50 μm.This has the following explanation. In the case wherein the raw materialwas thin, the battery could not exhibit a sufficient electronicconductivity, since the electrode support acts as a current collector.In the case where the raw material was thick, on the other hand, due tothe increased strength of the electrode support itself, when wound intoa cylindrical form to constitute a battery, there occurred distortion inthe active material layer and electrode support, whereby the activematerial became more liable to peel off caused by repeated charging anddischarging. Greater material thickness also causes reduced theoreticaldischarge capacity as can be seen from Table 2, which is a drawback toconstitute a battery of high energy density. Consequently, the thicknessof unprocessed metal sheet should be 80 μm or less in order to fullyachieve the effects of the present invention.

There was no conspicuous difference between battery C in which thepattern of forming protrusions and indentations was changed andbatteries M, N in the charge and discharge characteristics. Thisindicates that any of the patterns is effective. It may be said, though,only in close comparison between the differences of the patterns, thatthe patterns wherein protrusions and indentations are arrangedalternately in both longitudinal and transverse directions are better,from the fact that the utilization rate of the active material inbatteries M, N was greater. As for the material of metal sheets, similareffects were found in battery O using an electrode support made of purenickel plate to those of other examples made of nickel-plated rolledsteel sheet.

As described above, with the present invention, the batterycharacteristics of a coated type nickel positive electrode can besatisfactorily improved. The electrode support according to the presentinvention can be produced at lower cost by mechanically forming surfaceirregularities in a metal sheet or a punched metal sheet. The cost forelectrodes. can be thus reduced, and batteries can be manufactured atlower cost.

EXAMPLE 2

The effects of conductive agent were compared in the coated type nickelpositive electrode similarly to Example 1, using the electrode support cshown in Table 1 of Example 1. As the conductive agent, the effect ofadding cobalt oxide of higher order and nickel powder, and the effect ofcoating cobalt oxide of higher order on the surface of nickel hydroxidepowder constituting the active material were examined.

A paste mainly composed of active material was prepared in a similar wayto that in Example 1 without adding cobalt powder, and was coated on theelectrode support c. The electrode support c was then dried, compressed,and cut into the same dimensions to produce a coated nickel positiveelectrode for constituting a sealed battery of size C.

These nickel positive electrodes were combined with well-knownhydrogen-absorption alloy negative electrodes to configure fifteen typesof batteries P-1˜T-1, P-2˜T-2, U-1U˜5 in a similar way as shown inExample 1, and charge/discharge test was conducted under the sameconditions.

Table 3 shows the discharge capacity at 3rd cycle, utilization rate ofactive material (discharged at 0.2C.), theoretical discharge capacity ofnickel positive electrode, and filling density calculated from thevolume of the electrode of these batteries after charging anddischarging.

Cobalt oxide of higher order was either coated on the surface of nickelhydroxide or simply added during preparation of the paste with theamount varied. Nickel powder was added in the paste, with its amountvaried in each case. The amount of addition is indicated by the quantityin relation to 100 wt % of nickel hydroxide in Table 3.

TABLE 3 Filling Discharge at 3rd Amount of density cycle addition Amountof of Dis- Cond- coated on addition active charge Utiliza- Bat- uctiveactive mixed in material capacity tion tery agent material paste(mAh/cc) (mAh) rate (%) P-1 Cobalt 0 — 642 2549 74.2 oxide of higherorder Q-1 ditto 2 — 625 3087 92.3 R-1 ditto 5 — 600 3184 99.2 S-1 ditto10  — 558 3078 103.1 T-1 ditto 15  — 513 2877 104.8 P-2 ditto — 0 6692482 70.3 Q-2 ditto — 2 646 3138 90.8 R-2 ditto — 5 612 3202 97.8 S-2ditto — 10 565 2971 98.8 T-2 ditto — 15 518 2752 99.3 U-1 Nickel — 5 6302646 78.5 powder U-2 ditto — 10 598 2921 91.3 U-3 ditto — 20 567 292196.3 U-4 ditto — 25 535 2782 97.2 U-5 ditto — 30 503 2621 97.4

With respect to batteries P-1 to T-1 wherein the conductive high-ordercobalt oxide powder is formed on the surface of nickel hydroxide, theless the amount of addition of high-order cobalt oxide powder was, thegreater the filling density of active material was, while theutilization of active material decreased in accordance therewith. Thisindicates that the amount of cobalt oxide of higher order should be atleast 2 or more in relation to 100 wt % of nickel hydroxide, in order toconstitute a battery of high energy density. In battery T-1 wherein theamount of addition of cobalt oxide of higher order was 15 wt %, whilethe utilization increased, the filling density was reduced, as a resultof which the discharge capacity decreased. It is thus preferable to setthe amount of addition of cobalt oxide of higher order at 10 or less,since the use of relatively expensive cobalt oxide of higher order in agreat amount will lead to the increase in production cost of electrodes.

The similar trend was observed in batteries P-2˜T-2 wherein theconductive agent was added in the paste and in batteries U-1˜U-5 whereinnickel powder was used as the conductive agent in terms of fillingdensity and utilization rate of active material. It is thus assumed thatthe amount of addition of cobalt oxide of higher order is mostpreferably within the range 2˜10 wt % in relation to 100 wt % of nickelhydroxide, and that of nickel powder is within the range 10˜25 wt %.

As described above, the same effects were obtained in any of the caseswhere conductive high-order cobalt oxide powder was formed on thesurface of nickel hydroxide, and high-order cobalt oxide or nickelpowder was mixed in the paste as the conductive material, as the methodof adding cobalt powder as in Example 1.

It should be noted that the effects of the present invention can be alsoachieved in cases where cobalt oxide of higher order and nickel powderare admixed with each other in an appropriate amount, and not limited tothe cases where each conductive agent is solely used.

Although not shown in this Example, it was ascertained that the presentinvention would be also applicable in any of various electrode supportsaccording to the present invention shown in Example 1.

EXAMPLE 3

In this Example, the hydrogen-absorption alloy negative electrode of acylindrical sealed nickel-hydrogen storage battery produced using theelectrode support shown in Example 1 was examined. The perforated metalsheet c in Table 1 was selected for the electrode support. The processedmetal sheet c having the apparent thickness of 500 μm was preliminarilypassed through a roll press with a gap of 300 μm thereby to reduce theapparent thickness thereof to 300 μm.

A negative electrode was prepared using this perforated metal sheet.Carbon powder as a conductive agent and a binder of styrene-butadienewere added to alloy powder of the composition ofMmNi_(3.6)Mn_(0.4)Al_(0.3)Co_(0.7) hydrogen-absorption alloy of AB₅type, kneaded with water into a paste, which was coated on theperforated metal sheet c of which apparent thickness had been reduced to300 μm. The perforated metal sheet was then dried, compressed, and cutinto prescribed dimensions to obtain an electrode v according to thepresent invention.

As a comparative example, an electrode w was produced as a prior artelectrode by using a core material of normal punched metal such as theone used in metal sheet g in Table 1 having a thickness of 50 μm.

Sealed batteries were prepared using these two kinds of negativeelectrodes. Positive electrodes employed in these batteries wereobtained by mixing a well-known spherical nickel hydroxide powder withadditives such as zinc oxide, cobalt oxide, or cobalt hydroxide into apaste, filling this paste in a sponge-like nickel conductive porousmaterial, drying, compressing, and cutting into prescribed dimensions.For the separator, an unwoven fabric of sulfonated polypropylene havinga thickness of 0.12 mm was used. These positive electrode and negativeelectrode were wound in a spiral with the separator interposedtherebetween, and accommodated in an outer metal case of nickel-platediron. Positive and negative electrodes are connected by electric weldingto respective current collecting lead plates, thereby being connected topositive electrode terminal and negative electrode terminal,respectively.

An aqueous solution of potassium hydroxide of 1.3 specific gravitydissolved with lithium hydroxide 40 g/l was poured into the battery asthe electrolyte, after which the outer metal case was crimped with asealing cap thereby finishing a sealed battery. The battery is acylindrical sealed battery of size C with a nominal capacity of 2.8 Ah.

Hereinafter the battery using the electrode v of the present inventionwill be referred to as battery V, whereas the battery with the prior artelectrode w as battery W.

The characteristics of these batteries were examined, and in particular,they were compared with respect to the high-rate dischargecharacteristic and cycle life characteristic. First, the batteries werecharged for 15 hours at 0.1C. at 20° C., and then left for one hour andthereafter discharged at 0.2C. until the battery voltage decreased to1.0V. Three cycles of this test were repeated under the same conditions.Next, under similar charging conditions, the fourth-cycle test wasconducted with a discharge current of 5C., and the fifth-cycle test wasconducted with a discharge current of 10C., to compare the high-ratedischarge characteristics.

Also, for the sixth and subsequent cycles, the cycle life test wasconducted by charging at 0.5C. for 2.5 hours and discharging at 0.5C.until the battery voltage decreased to 1V to compare the structure ofthe negative electrode and the cycle life characteristics. The resultsare shown in TabIe 4.

According to the evaluation of these characteristics, all of thebatteries exhibited favorable battery characteristics until the thirdcycle and they all satisfied the nominal capacity. The results ofperformance evaluation in the high-rate discharge characteristic testare indicated by the discharge voltage (intermediate voltage) and theutilization rate. The number of cycles in cycle life test is shown bythe number of cycles at which the discharge capacity of the battery hasreached 70% or lower of the initial capacity.

TABLE 4 High-rate discharge characteristic test 5C Discharge 10CDischarge Cycle life test Battery V 1.18 V, 87% 1.14 V, 83% 650 cyclesBattery W 1.16 V, 74% 1.11 V, 68% 480 cycles

As can be seen from Table 4, it was ascertained that the battery Vaccording to the present invention had superior performance both inhigh-rate discharge characteristic and cycle life characteristic, incomparison with the battery W of the prior art. These differences inperformance shown in Table 4 were caused solely by employing differentelectrode supports in the negative electrode when constituting thebattery. The high-rate discharge characteristic and cycle lifecharacteristic can be thus remarkably enhanced by using the electrodesupport of the present invention in the negative electrode, too.

As described above, with the present invention, in comparison with flatpunched metal used as an electrode support of a coated-type nickelelectrode, the battery characteristics can be improved. Further, theelectrode according to the present invention can be produced at lowercost than those electrodes with a three-dimensionally foamed porousnickel substrate or fabric-type nickel processed into felt. Theelectrode cost can thus be reduced. Furthermore, the present inventionis obviously applicable not only to the coated-type nickel electrode andhydrogen-absorption alloy electrode described above with reference tothe embodiments but also to other coated-type electrodes including azinc electrode and a cadmium electrode, or to positive and negativeelectrodes of non-aqueous electrolyte batteries such as lithium-ionstorage battery and lithium-polymer storage battery.

Industrial Applicability

The adhesion between the active material layer and the electrode supportof an electrode for secondary batteries using a metal sheet can beimproved by forming minute surface irregularities thereby to provide athree-dimensional structure in the metal sheet by a mechanical method.As a result, remarkable improvements in battery characteristics can beachieved, such as enhancement of the utilization of active material,improvement in the high-rate discharge characteristic, and elongation ofelectrode life. Moreover, the electrode support according to the presentinvention can be fabricated by a simple mechanical method at a far lowercost in comparison with foamed metal-type electrodes. The presentinvention is thus of high industrial value.

What is claimed is:
 1. A non-sintered type electrode comprising anelectrode support made of a metal sheet having minute surfaceirregularities on which is coated or pressed an active material forbattery or an active material retaining medium, said surfaceirregularities are formed by a mechanical method such that protrusionsand indentations are configured with a center-to-center pitch (P) in therange of from 50 to 300 μm, and such that the apparent thickness of theelectrode support after forming the surface irregularities is at leastthree times as large as the thickness before the formation of thesurface irregularities.
 2. The non-sintered type electrode according toclaim 1, wherein each of the protrusions and indentations is formed in asubstantially tapered shape.
 3. The non-sintered type electrodeaccording to claim 1, wherein each of the protrusions and indentationsis formed in a substantially conical shape.
 4. The non-sintered typeelectrode according to claim 1, wherein the apparent thickness of theelectrode support after forming the surface irregularities is at leastfive times as large as the thickness before the formation of the surfaceirregularities.
 5. The non-sintered type electrode according to claim 1,wherein the electrode support is constructed of a punched metal of whichmaterial thickness before the formation of the surface irregularities isin the range of from 10 to 80 μm.
 6. The non-sintered type electrodeaccording to claim 1, wherein the electrode support is constructed of anon-punched metal sheet of which material thickness before the formationof the surface irregularities is in the range of from 10 to 80 μm. 7.The non-sintered type electrode according to claim 1, wherein the metalsheet with the surface irregularities has innumerable minute holesformed by piercing through the tops of the protrusions.
 8. Thenon-sintered type electrode according to claim 1, wherein thecenter-to-center pitch (P) of the protrusions and indentations is in therange of from 100 to 200 μm.
 9. The non-sintered type electrodeaccording to claim 1, wherein the apparent thickness of the electrodesupport is more than 400 μm.
 10. The non-sintered type electrodeaccording to claim 1, wherein the electrode support is constructed ofany one of steel sheet, nickel-plated steel sheet, and nickel sheet. 11.The non-sintered type electrode according to claim 1, wherein theprotrusions and indentations of the surface irregularities are formed atrandom with the respective numbers of protrusions and indentations inranges of from 80 to 20% and from 20 to 80% per unit area.
 12. Thenon-sintered type electrode according to claim 1, wherein theprotrusions and indentations of the surface irregularities are formed inorder such that the protrusions and indentations are arrangedalternately in one direction.
 13. The non-sintered type electrodeaccording to claim 1, wherein the protrusions and indentations of thesurface irregularities are formed in order such that the protrusions andindentations are arranged alternately in both longitudinal andtransverse directions.
 14. The non-sintered type electrode according toclaim 1 for use as a nickel positive electrode of an alkaline storagebattery, wherein the active material comprises nickel hydroxide powderof which surface is coated with cobalt oxide of higher order at the rateof from 2 to 10 wt % in relation to 100 wt % of nickel hydroxide. 15.The non-sintered type electrode according to claim 1 for use as a nickelpositive electrode of an alkaline storage battery, wherein the activematerial comprises nickel hydroxide powder in which is contained atleast cobalt oxide of higher order or nickel in such a way as to contactthe nickel hydroxide powder.
 16. The non-sintered type electrodeaccording to claim 15, wherein the cobalt oxide of higher order issolely contained at the rate of from 2 to 10 wt % in relation to 100 wt% of nickel hydroxide.
 17. The non-sintered type electrode according toclaim 15, wherein the nickel is solely contained at the rate of from 10to 25 wt % in relation to 100 wt % of nickel hydroxide.
 18. Thenon-sintered type electrode according to claim 1 for use as ahydrogen-absorption alloy negative electrode of an alkaline storagebattery, wherein the active material retaining medium compriseshydrogen-absorption alloy powder of which surface is in contact with aconductive agent selected from the group consisting of nickel, copper,and carbon, the conductive agent being contained at the rate of from 0.5to 10 wt % in relation to 100 wt % of hydrogen-absorption alloy powder.19. A method of manufacturing a non-sintered type electrode comprisingforming minute irregularities on the surface of a metal sheet bypress-machining using metal molds such that protrusions and indentationsare configured with a center-to-center pitch (P) in the range of from 50to 300 μm and such that the apparent thickness of the metal sheet afterforming the surface irregularities is at least three times as large asthe thickness before the formation of the surface irregularities,thereby fabricating an electrode support, and an active material forbattery or an active material retaining medium is coated or pressed onthe surface of the thus obtained electrode support.
 20. The method ofmanufacturing a non-sintered type electrode according to claim 19,wherein in the process of press-machining the metal sheet with metalmolds, minute holes are formed simultaneously with the formation of thesurface irregularities by piercing through the tops of the protrusions.21. The method of manufacturing a non-sintered type electrode accordingto claim 19, wherein each of the protrusions and indentations is formedin a substantially tapered shape.